Anatomical terms of location

Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans.

While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories.

Unique anatomical terminology is used to describe humans and other animals. Because of differences in the way humans and other animals are structured, different terms are used depending on the neuraxis and whether an animal is a vertebrate and invertebrate.

Standardized anatomical and zoological terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists to precisely delineate and communicate information about animal bodies and their component organs, even though the meaning of some of the terms often is context-sensitive.[1]

The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used to describe location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way.

For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position. Thus what is on "top" of a human is the head, whereas the "top" of a dog may be its back, and the "top" of a flounder could refer to either its left or its right side.

For invertebrates, standard application of locational terminology often becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not even bilaterally symmetrical. In these species, terminology depends on their type of symmetry (if any).

A jellyfish of the Chrysaora species. Like other animals, its appendages move, and in this image are not in a standard anatomical position. In anatomical position, the proximodistal axis (labelled) is straight, and the point labelled "distal end" neither to the left or the right of the jellyfishes' main axis.

Because animals can change orientation with respect to their environment, and because appendages like limbs and tentacles can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position. All descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. This helps avoid confusion in terminology when referring to the same organism in different postures.

In humans, this refers to the body in a standing position with arms at the side and palms facing forward (thumbs out). While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing.

Many anatomical terms can be combined, either to indicate a position in two axes simultaneously or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position that is both anterior and lateral to the body axis (such as the bulk of the pectoralis major muscle). In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.[2]

There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. Generally the modifier term is truncated and an "o" is added in prefixing it to the qualified term. For example a view of an animal from an aspect dorsal and lateral might be called "dorsolateral". Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral". Such terms sometimes used to be hyphenated, but the modern tendency is to omit the hyphen. There is however little basis for any strict rule to interfere with choice of convenience in such use.[3]

The sagittal plane is a plane parallel to the sagittal suture, All other sagittal planes (referred to as parasagittal planes) are parallel to it. It is also known as a "longitudinal plane".[4] The plane is an Y-Z plane, perpendicular to the ground.

The median plane or midsagittal plane is in the midline of the body, and divides the body into left and right (sinister and dexter) portions. [4] This passes through the head, spinal cord, navel and, in animals, the tail. [4] The median plane can also refer to the midsagittal plane of other structures, such as a digit.

The frontal plane or coronal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical. A longitudinal plane is any plane perpendicular to the transverse plane. The coronal plane and the sagittal plane are examples of longitudinal planes.

A transverse plane, also known as a cross-section, divides the body into cranial and caudal (head and tail) portions.

In human anatomy

A transverse (also known as horizontal) plane is an X-Y plane, parallel to the ground, which (in humans) separates the superior from the inferior, or put another way, the head from the feet.

A coronal (also known as frontal) plane is a X-Z plane, perpendicular to the ground, which (in humans) separates the anterior from the posterior, the front from the back, the ventral from the dorsal.

Notes:
(1) Fairly common usage.
(2) Uncommon usage.
(3) Equivalent to one-half of the left-right axis.
(The terms "intermediate", "ipsilateral", "contralateral", "superficial", and "deep", while indicating directions, are relative terms and thus do not properly define fixed anatomical axes. Also, while the "rostrocaudal" and anteroposterior directionality are equivalent in a significant portion of the human body, they are different directions in other parts of the body.)

To begin with, distinct, polar-opposite ends of the organism are chosen. By definition, each pair of opposite points defines an axis. In a bilaterally symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles—the x, y, and z axes familiar from three-dimensional geometry.

As humans are approximately bilaterally symmetrical organisms,[5] anatomical descriptions usually use the same terms as those for vertebrates and other members of the taxonomic group Bilateria. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally symmetrical organisms.

As with other vertebrates, two of the most obvious extremes are the "top" and the "bottom" of the organism. In standard anatomical position in humans, these correspond to the head and feet. The head end of the body is referred to as the superior end (Latinsuperior: "above"), while the feet end is the inferior end (Latin inferior: "below"). An axis formed joining the two is the superior-inferior axis.[6][7]

As with other vertebrate terminology, there are synonymous terms for superior and inferior (Table 3). The terms cranial and cephalic are often used. Both "cranial" and "cephalic" are used to refer to the skull, with "cranial" being used more commonly. The term rostral is rarely used in human anatomy, apart from embryology, and refers more to the front of the face than the superior aspect of the organism. This term is more applicable in organisms with longer heads, such as equids.[7] Similarly, the term caudal is occasionally used in human anatomy,[7] and the cranio-caudal axis is occasionally encountered. Generally, this use would be used with respect to only the head and the main body (trunk), and not when considering the limbs. In the limbs of most animals, the terms cranial and caudal are used in the regions proximal to the carpus (the wrist, in the forelimb) and the tarsus (the ankle in the hindlimb). Objects and surfaces closer to or facing toward the head are cranial; those facing away or farther from the head are caudal.

As with vertebrate directional terms, superior and inferior can be used in a relative sense in humans, but can not be uniformly applied to other organisms with varying normal anatomical positions. For example, the shoulders are superior to the navel, but inferior to the eyes in humans.

In humans, anterior refers to the front of the individual, and is synonymous with ventral, other than in the head. Similarly, posterior, refers to the back of the subject, and is synonymous with dorsal, other than in the head. [8] When referring to the body as a whole the terms dorsal and ventral are used infrequently in human anatomy. However, they are applied commonly in referring to limb position.[9] "Anteroposterior" is the preferred term for describing the axis connecting the front and the back in humans.[6][7] For example, the anteroposterior diameter (AP diameter) of the chest is the width of the chest as measured from front to back.

Anterior and posterior can also be used as relative terms. Thus, the eyes are posterior to the nose, but anterior to the back of the head in humans.

Lateral refers to the sides, as in 'left lateral' and 'right lateral'. Medial refers to the middle. [8] Left and right lateral are used in the same sense as they are in other vertebrates, referring to the individual's left and right. [8] The left-right axis is rarely used in medicine; instead, the mediolateral axis is used almost exclusively.[6][7]

As in other vertebrates, the terms proximal and distal are used to describe parts of a feature that are respectively close to or distant from the main mass of the body,[8] or some other point of reference. The point of reference varies, but is normally the point of attachment to the main body for limbs and appendages, and a point considered to represent the origin for other features. For example the side of a tooth furthest from the center of the mouth is its distal side. However, other terms are used for direction in the appendages, given the unique position of the limbs (in standard anatomical position) in humans.[10] For example, the segments of the digestive system closest to the mouth are termed proximal, as opposed to those closest to the anus, which are termed distal.

Specialized terms are used to describe location on appendages, parts that have a point of attachment to the main trunk of the body. Structures that are close to the point of attachment of the body are proximal or central, while ones more distant from the attachment point are distal or peripheral. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends; the proximal end of the urethra is attached to the bladder.

Volar refers to the underside, for both the palm and the sole; thus, palmar and plantar are hyponyms of volar. For example, volar pads are those on the underside of hands, fingers, feet, and toes.

The terms valgus and varus are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5–10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.

Latin convention

Commonly when, for example, one anatomical feature is nearer the caudal end than another, one may use an expression such as "nearer the caudal end" or "caudal to". However, an unambiguous and concise convention is to use the Latin suffix -ad, meaning "towards", or sometimes "to". [8] So for example, "distad" means "in the distal direction", and "distad of the femur" means "beyond the femur in the distal direction". The suffix may be used very widely, as in the following examples: anteriad (towards the anterior), apicad (towards the apex), basad (towards the basal end), caudad, centrad, cephalad (towards the cephalic end), craniad, dextrad, dextrocaudad, dextrocephalad, distad, dorsad, ectad (towards the ectal, or exterior, direction), entad (towards the interior), laterad, mediad, mesad, neurad, orad, posteriad, proximad, rostrad, sinistrad, sinistrocaudad, sinistrocephalad, ventrad.[11]

Structures may be described as being at the level of a specific spinal vertebra, depending on the section of the vertebral column the structure is at. The position is often abbreviated. For example, a structures at the level of the fourth cervical vertebra may be abbreviated as "C4", at the level of a thoracic vertebra "T4", at the level of a lumbar vertebra "L3". Because the sacrum is fused, it is not often used to provide location.

Anatomical "lines", theoretical lines drawn through structures, are also used to describe anatomical location. For example:

The midaxillary line, a line running vertically down the surface of the body passing through the apex of the axilla (armpit). Parallel are the anterior axillary line, which passes through the anterior axillary skinfold, and the posterior axillary line, which passes through the posterior axillary skinfold.

The mid-clavicular line, a line running vertically down the surface of the body passing through the midpoint of the clavicle.

The mid-pupillary line, a line running vertically down the face through the midpoint of the pupil when looking directly forward.

Fields such as osteology, palaeontology and dentistry apply special terms of location to describe the mouth and teeth. This is because although teeth may be aligned with their main axes within the jaw, some different relationships require special terminology as well; for example teeth also can be rotated, and in such contexts terms like "anterior" or "lateral" become ambiguous.[12][13]

Buccal (Latin bucca; cheek): on the side that faces the cheek

Buccolingual: the position or the axis between the tongue and cheek, as in the position of the molar teeth

Distal: away from the mandibular symphysis

Facial: on the side away from the tongue

Incisal: referring to the occlusal or biting edge of an incisor tooth

Interproximal: Between adjacent teeth, such as the area of contact between two adjacent teeth

Labial (Latin labium; lip): facing the lips

Labiolingual: the position or the axis between the tongue and the lips, as in the position of the incisor teeth

In standard anatomical position, the palms of the hands face anteriorly. To avoid confusion, several unique terms are used.

For improved clarity, the directional term palmar (Latin: palma; palm of the hand) is usually used for the anterior of the hand, and dorsal is used to describe the back of the hand. Thus, by connecting the extremes, dorsopalmar axis is formed. Most commonly, "dorsopalmar" is used when describing the hand, although it is sometimes applied to the arm as a whole (see Fig. 12). For the third axis, the mediolateral axis suffices, although if referring to the limb alone, "medial" may refer to the centre of the arm itself. This replaces "anterior" and "posterior", which can also be used — "anterior" can be used to describe the palm of the hand, and "posterior" can be used to describe the back of the hand and arm.

In the forearm, for clarity, the sides are named after the bones. Structures closer to the radius are radial, structures closer to the ulna are ulnar, and structures relating to both bones are referred to as radioulnar. Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).

Most terms of anatomical location are relative to linear motion (translation) along the X- Y- and Z-axes, but there are other degrees of freedom as well, in particular, rotation around any of those three axes.

Anteversion and retroversion are complementary anatomical terms of location, describing the degree to which an anatomical structure is rotated forwards (towards the front of the body) or backwards (towards the back of the body) respectively, relative to some datum position. The terms also describe the positioning of surgical implants, such as in arthroplasty.

Anteversion refers to an anatomical structure being tilted further forward than normal, whether pathologically or incidentally. For example there may be a need to measure the anteversion of the neck of a bone such as a femur.[14] For example, a woman's uterus typically is anteverted, tilted slightly forward. A misaligned pelvis may be anteverted, that is to say tilted forward to some relevant degree.

Retroversion is rotation around the same axis as that of anteversion, but in the opposite sense, that is to say, tilting back. A structure so affected is described as being retroverted. As with anteversion, retroversion is a completely general term and can apply to a backward tilting of such hard structures as bones, soft organs such as uteri, or surgical implants.

All vertebrates (including humans) have the same basic body plan — they are bilaterally symmetrical.[5] That is, they have mirror-image left and right halves if divided down the centre.[15] For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate) organisms as well.

Two obvious end-points of an organism are the nose and tail. Anatomically, the nose is referred to as the anterior end (Latin: ante; before), with the opposite end of the organism being the posterior end (Latin: post; after).

By drawing a line connecting these two points defines the "anteroposterior axis", often abbreviated to the "AP axis". As well as defining the anteroposterior axis, the terms "anterior" and "posterior" also define relative positions along the axis. For example, in many fish the gill openings are posterior relative to the eyes, but anterior to the tail.

Distal to the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, distal to the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. For example, the top of a dog's paw is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.

Cranial and caudal

In organisms like vertebrates, that have distinct heads, the anterior end is sometimes referred to as the rostral end (Latin: rostrum; beak), the cranial end (Greek κρανίον (kranion); skull), or the cephalic end (Greek: κεφάλι (kephalē); head).[15] Another term for posterior is caudal (Latin: cauda; tail), though in humans this refers to the feet i.e. inferior rather than posterior)—a term that strictly applies only to vertebrates, and therefore less preferred, except in veterinary medicine where these terms are standard,[15][16][17] as accepted for example in the Nomina Anatomica Veterinaria.[18] In veterinary medicine, caudo-cranial is preferred between head and tail, and rostro-caudal between nose and neck. Less-used synonyms would be rostrocaudal or cephalocaudal axes (see Table 1). In the horse, for example, the eyes are caudal to the nose and rostral to the back of the head.

Caudal (Latin: cauda; tail): of, at, or near the tail or the posterior end of the body. In the human case, toward the bottom of the feet (also the "tail" of the spinal cord, and body).

Rostral (Latin: rostrum; beak or nose): situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.

Rostral and Caudal shown on a human skull, this division is due to the neuraxis of humans

In humans, the directions "rostral" and "caudal" often become confused with anterior and posterior, or superior and inferior. The difference between the two is most easily visualized when looking at the head, as can be seen in the image to the right. From the most caudal of positions in the nervous system (of a person) to a nearby, rostral area, it is equally accurate to say the area in question is rostral as to say it is superior. However, in the frontal lobes of the telencephalon, to say an area is rostral to a nearby area is equivalent to saying it is anterior. Those two lines lie on planes perpendicular to one another. This occurs, as becomes clear in the diagram, due to the intuitive yet curious curving "C" shape of rostrocaudal directionality when discussing the human brain.

These two terms refer to front/belly (ventral) and back (dorsal) of an organism.

The dorsal (Latin: dorsum, "back") surface of an organism refers to the back. [19] For example, the dorsal fin.

The ventral (Latin: venter, "belly") surface refers to the front, or lower side, of an organism.[19]

By connecting the outermost points the dorsoventral axis is formed.[19] This is commonly abbreviated to DV axis. The DV axis, is perpendicular the AP axis at all times.[19] The terms "dorsal" and "ventral" are also used to describe relative positions. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin.

The last axis, by geometric definition, must be at right angles to the anteroposterior and dorsoventral axist.. The left side and right side of the organism are the outermost points between the two "sides" of the organism. When connected, these points form the "left-right axis". The "left" and "right" refer to the sides of the organism, and not those of the observer. In Latin, this is called the "dextro-sinistral axis" (Latin: dexter; "right", Latin: sinister; "left").

As with the other directions, the terms can be used as relative terms, to describe locations along the left-right axis. Thus, in Fig. 2 the dorsal fin is right of the left pectoral fin, but is left of the right eye. However, as left and right sides are mirror images, use like this tends to be somewhat confusing, as structures are duplicated on both sides (i.e., in the previous example, there is both a right eye and a left eye, forcing one to specify which is used as a reference).

To counter this clumsiness of use, the directional term lateral (Latin: lateralis; "to the side") is used as a modifier for both sides, yielding the left lateral and right lateral sides. As an opposite to lateral, the term median (Latin: medius; "middle") is used to define a point in the centre of the organism (where the left-right axis intersects the midsagittal plane —see below), and the term medial means "toward the median plane". Thus, rather than "left-right" axis and its inherent clumsiness of usage, the term mediolateral axis is frequently used. Sometimes this is abbreviated to ML axis.[15] In proper usage, the ML axis is a half-axis; in practice, its use is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. Thus, in Fig. 2 the gills are medial to the operculum, but lateral to the heart.

The use "mediolateral" is strictly used to describe relative position along the left-right axis, to avoid confusion with the terms "superficial" and "deep".

Contralateral (Latin: contra; against): on the side opposite to another structure. For example, the left arm is contralateral to the right arm, or the right leg.

Ipsilateral or, rarely, homolateral (Latin: ipse; self/same + latus-eris, "side"): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.

"Proximal" and "distal" redirect here. For demonstrative words, see Demonstrative.

The term proximal (Latin: proximus, "nearest") describes where the appendage joins the body, and the term distal (Latin: distare, "to stand away from") is used for the point furthest from the point of attachment to the body. Since appendages often move independently of the main body, and therefore change position with respect to, these separate directional terms are used when describing them.

As noted above, the standard AP, DV, and ML directional axes can cause some confusion when describing parts of the body that can change position relative to the main body. This is particularly true when considering appendages such as fins, limbs and indeed any structure that extends and can at least potentially move separately from the main body. [a]

By connecting the two points, the proximodistal axis is created. (The abbreviation AB axis is occasionally, but not commonly, used.) As before, the terms "proximal" and "distal" can be used as relative terms to indicate where structures lie along the proximodistal axis. Thus, the "elbow" is proximal to the hoof, but distal to the "shoulder" in Figs. 3 and 4.

Choosing terms for the other two axes perpendicular to the proximodistal axis could be variable, as they would also depend on the position of the limb. For that reason, when considering any organism, the other two axes are considered to be relative to the appendage when in standard anatomical position. This is roughly defined for all organisms, as in the normal position when at rest and not moving. For tetrapodvertebrates, this includes the caveat that they are standing erect and not lying down. Thus, the fish in Fig. 2, and the horse in Figs. 3 and 4 are in standard anatomical position.

Bone

Similar to appendages that branch out from the body, the directions of blood vessels may be labeled with the terms ostial, referring to the ostium or opening where the vessel branches off, and distal, referring to the extreme end away from the branch point, may be used.

These two terms relate to the distance of a structure from the surface of an organism.

Deep: further away from the surface of the organism. For example, the external abdominal muscular layers are deep to the skin, but superficial to the intestines. In English "deep" is one of the few terms from the vernacular that have become prevalent in anatomical terminology; the anglicised Latin term to match "superficial" would have been "profound" (Latin: profundus, "due to depth"), and in other languages the equivalent term usually is derived from profundus (e.g. profond, meaning deep, in French).

Superficial (Latin: superfacies; "at the surface or face"): near the outer surface of the organism. For example, skin is superficial to the muscle layer. The opposite is "deep", or "visceral".

Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include:

Axial (Latin: axis, from Greek: axōn "axle"): around the central axis of the organism or the extremity. Two related terms, "abaxial" and "adaxial", refer to locations away and toward the central axis of an organism, respectively.

Intermediate (Latin: intermedius, from inter, between and medius, middle): between two other structures. For example, the navel is intermediate to the left arm and the contralateral (right) leg.

Parietal (Latin: paries, "wall"): pertaining to the wall of a body cavity. For example, the parietal peritoneum is the lining on the inside of the abdominal cavity. Parietal can also refer specifically to the parietal bone of the skull or associated structures.

Visceral (Latin: viscus, "internal organs, flesh"): associated with organs within the body's cavities. For example, the stomach is covered with a lining called the visceral peritoneum. Viscus can also be used to mean "organ". For example, the stomach is a viscus within the abdominal cavity.

The terms of zootomy and anatomy came into use at a time when all scientific communication took place in Latin. In their original Latin forms the respective meanings of "anterior" and "posterior" are in front of (or before) and behind (or after), those of "dorsal" and "ventral" are toward the spine and toward the belly, and those of "superior" and "inferior" are above and below. From these meanings it can be seen that in the most general terms the anterior/posterior axis is oriented to the direction of forward motion, the dorsal/ventral axis is oriented to the anatomy of the vertebrate torso, and the superior/inferior axis is oriented to gravity.

For almost all vertebrates, including almost all bipeds, these axes all provide a consistent reference for anatomical positions across species—with the inferior/superior axis being roughly the same as the dorsal/ventral axis, and therefore redundant. Humans, however, have the rare property of having a torso oriented perpendicular to their direction of forward motion—while their head orientation remains consistent with other vertebrates on this axis. This makes the dorsal/ventral axis on humans redundant with the anterior/posterior axis, and the inferior/superior axis necessary. Because of this difference with humans, the anterior/posterior and inferior/superior axes are inconsistent between humans and other vertebrates in torso anatomy but consistent in head anatomy. As all three of these axes are used in the naming of anatomical structures, and most human anatomical structures are shared by other animals, these differences can lead to considerable confusion. For example, in the naming of brain structures, the non-human context of the dorsal/ventral axis was used. Therefore, in human anatomy, "dorsal" can refer to two different (perpendicular) directions—the posterior direction in the context of the torso, and the superior direction in the context of the brain. Confusingly then, the "dorsal" direction in the human brain, being perpendicular to the "dorsal" direction in the human torso, conflicts with the direction that might be inferred from the literal Latin meaning of "dorsum": a back or a mountain ridge.[20]

Most animals, furthermore, are capable of moving relative to their environment. So while "up" might refer to the direction of a standing human's head, the same term ("up") might be used to refer to the direction of the belly of a supine human. It is also necessary to employ some specific anatomical knowledge in order to apply the terminology unambiguously: For example, while the ears would be superior to (above) the shoulders in a human, this fails when describing the armadillo, where the shoulders are above the ears. Thus, in veterinary terminology, the ears would be cranial to (i.e., "toward the head from") the shoulders in the armadillo, the dog, the kangaroo, or any other terrestrial vertebrate, including the human. Likewise, while the belly is considered anterior to (in front of) the back in humans, this terminology fails for the flounder, the armadillo, and the dog. In veterinary terms, the belly would be ventral ("toward the abdomen") in all vertebrates.

While it would be possible to introduce a system of axes that is completely consistent between humans and other vertebrates by having two separate pairs of axes, one used exclusively for the head (e.g., anterior/posterior and inferior/superior) and the other exclusively for the torso (e.g., dorsal/ventral and caudal/rostral, or "toward the tail"/"toward the beak"), doing so would require the renaming of very many anatomical structures.

For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).

Figure 3: Directional axes in the tetrapodvertebrateEquus caballus (a horse). The axis between cranial and caudal is the Cr-Cd axis, and between the dorsal and ventral is the D-V axis. (Left-right axis not shown; image shows the right side of the organism.)

Figure 4: Different directional AP axes in three body segments of a horse). Axis (A) (in red) shows the AP axis of the tail, (B) shows the AP axis of the neck, and (C) shows the AP axis of the head.

Both images have been rendered with the same height, and a total width of 320px

Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally symmetrical organism, whether vertebrate or invertebrate. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form, which includes most tetrapods (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.

When considering any one segment, the dorsoventral axis is perpendicular to the AP axis. Thus, in Fig. 4, the DV axis of the tail would run from the "back" of the tail (posterior end of the trunk), to the "underside" of the tail (near the legs) — nearly parallel to the AP axis of the main body.

As a rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. In alternative manner, to avoid confusion, AP, DV, and ML terms are used strictly in relation to the main body, and the terms proximal and distal are used for body segments such as the head, neck, and tail (see below).

To avoid this confusion, in veterinary medicine, the terms anterior, posterior, superior, and inferior are in general avoided except for certain structures within the head.[18] By using the terms cranial, caudal, dorsal and ventral, all tetrapod organisms (including bipeds) can be described uniformly.

The large variety of body shapes present in invertebrates presents a difficult problem when attempting to apply standard directional terms. Depending on the organism, some terms are taken by analogy from vertebrate anatomy, and appropriate novel terms are applied as needed. Some such borrowed terms are widely applicable in most invertebrates; for example proximal, literally meaning "near" refers to the part of an appendage nearest to where it joins the body, and distal, literally meaning "standing away from" is used for the part furthest from the point of attachment. In all cases, the usage of terms is dependent on the bauplan of the organism.

For example, especially in organisms without distinct heads for reasons of broader applicability, "anterior" is usually preferred.[15][16][17]

In organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in spherically symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive.

Figure 6: Four individuals of Phaeodactylum tricornutum, a diatom with a fixed elongated shape.

In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms are planktonic (free-swimming) protists, and are nearly always viewed on microscope slides, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present.[17]

Figure 7: Organisms where the ends of the long axis are distinct. (Paramecium caudatum, above, and Stentor roeseli, below.)

Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed.[17] Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface).[21]

Organisms that are attached to a substrate, such as sponges, or some animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Latin: basis, "support/foundation"), whereas the end furthest from the attachment is referred to as the apical end (Latin: apex, "peak/tip"). Thus, by joining the two ends, an apical-basal axis is formed. Transverse axes may be defined indifferently in any direction perpendicular to this axis, as there is no symmetry present.

Cnidarians (jellyfish, sea anemones and corals) have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron).[17] For this reason, the end of the organism with the mouth is referred to as the oral end (Latin: oris, "mouth"), and the opposite surface is the aboral end (Latin: ab-, prefix meaning "away from"). By joining the polar opposite oral and aboral ends, an oral-aboral axis is formed.

Unlike vertebrates, cnidarians have no other distinctive axes. "Lateral", "dorsal", and "ventral" have no meaning in such organisms, and all can be replaced by the generic term peripheral (Latin: peri-, "around"). Medial can be used, but in the case of radiates indicates the central point, rather than a central axis as in vertebrates. Thus, there are multiple possible radial axes and medio-peripheral (half-) axes. However, it is noteworthy that some biradially symmetrical comb jellies do have distinct "tentacular" and "pharyngeal" axes[22] and are thus anatomically equivalent to bilaterally symmetrical animals.

Two specialized terms are useful in describing views of arachnid legs and pedipalps. Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.[23]

Because of the unusual nature and positions of the eyes of the Araneae (spiders), and their importance in taxonomy, evolution and anatomy, special terminology with associated abbreviations has become established in arachnology. Araneae normally have eight eyes in four pairs. All the eyes are on the carapace of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxa. In some taxa not all four pairs of eyes are present, the relevant species having only three, two, or one pair of eyes. Some species (mainly troglobites) have no functional eyes at all.

In what is seen as the likeliest ancestral arrangement of the eyes of the Araneae, there are two roughly parallel, horizontal, symmetrical, transverse rows of eyes, each containing two symmetrically placed pairs, respectively called: anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME).

As a rule it is not difficult to guess which eyes are which in a living or preserved specimen, but sometimes it can be. Apart from the fact that in some species one or more pairs may be missing, sometimes eyes from the posterior and anterior rows may be very close to each other, or even fused. Also, either one row or both might be so grossly curved that some of the notionally anterior eyes actually may lie posterior to some of the eyes in the posterior row. In some species the curve is so gross that the eyes apparently are arranged into two anteroposterior parallel rows of eyes.

Aspects of spider anatomy; This aspect shows the mainly prolateral surface of the anterior femora, plus the typical horizontal eye pattern of the Sparassidae

Typical arrangement of eyes in the Lycosidae, with PME being the largest